Interpretive Summary: We examined the effects of an invasive weed, Russian knapweed, on the germination and seedling of four native grasses found in shortgrass steppe rangeland (blue grama, western wheatgrass, prairie junegrass, and sanddropseed). Both greenhouse and field studies were conducted. Greenhouse studies were used to examine the effects of Russian knapweed roots, litter or roots+litter on seed germination and seedling survival of each grass species. Field studies were conducted at three sites in Colorado. Germination and seedling survival were decreased by the presence of Russian knapweed in both the greenhouse and field for all species except western wheatgrass. These results support previous studies showing that western wheatgrass may have a large potential for successful when revegetating rangelands following invasion by weeds such as Russian knapweed. These species-specific responses to the presence of Russian knapweed may explain, at least in part, differences in susceptibility and recovery of sites with different native grass species composition.

Technical Abstract:
Our objective was to evaluate the effects of an invasive perennial forb, Acroptilon repens (Russian knapweed) on seed germination and seedling survival of four native grass species that are important in semiarid grasslands of North America. Greenhouse experiments and field studies conducted at three sites in Colorado were used to examine the response by four perennial grasses: Bouteloua gracilis, Koelaria cristata, Sporobolus cryptandrus, and Agropyon smithii to A. repens. In the greenhouse, seeds of each species were sown in pots that contained either live A. repens roots, litter on the soil surface, or both roots and litter. Field transects were placed inside stands of A. repens with adjacent control transects in the surrounding grass-dominated community. Germination and initial survival were decreased by the presence of A. repens roots for K. cristata (35%), B. gracilis (31%), and S. cryptandrus (44%) in the greenhouse, and 57, 32, and d36%, respectively, in the field. Root weight was decreased by the presence of A. repens roots for both B. gracilis and K. cristata by more than 55% in the greenhouse. A. smithii survival and growth was unaffected by A. repens in either the greenhouse or the field. These species-specific responses to the presence of A. repens may explain, at least in part, differences in susceptibility and recovery of sites with different native grass species composition.